Electron discharge device



June 3, 1941. A, SKELLETT 2,244,318

ELECTRON DISCHARGE DEVICE Filed Dec. 31, 1937 ANODE' POTENTIAL Pl K I 3BELOW GROUND ABOVE ROUND V a C4 THODE TO ANODE VOLTAGE INVENTOR P2AMSKELLETT A T TORNE V Patented June 3, 1941 ELE " ON DISC GE DEVKCEAlbert M. Skellett, ldison, N. 3., assignor to Bell 'ielephoneLaboratories,

Incorporated, New

' 11 Claims.

This invention relates to electron discharge devices-utilizing secondaryelectron emission and more particularly to such devices in whichrepeated electron multiplication is obtained by secondary electronemission from the .same cathode.

An object of this invention is to provide an improved electron dischargedevice wherein electron multiplication takes place by repeatedly andsuccessively causing electrons to bombard a cathode, capable of emittingsecondary electrons, with sumcient force to cause increased emissions bythe impacts.

Another object is to provide an improved photoelectric tube in which thephotoelectric current is amplified by the cathode of the tube emittingsecondary electrons due to successive electron bombardment of the samecathode.

A further object is to provide an improved oscillation generator inwhich the cathode of an electron discharge device successively emitssecondary electrons.

An electron discharge device illustrative of this invention utilizessecondary electron emission in such a way that electrons emitted fromthe convex surface of a hollow cylindrical cathode travel in loopedpaths and return to the cathode to cause increased emission bybombardment. The control is both electrostatic and magnetic. One form ofthe device comprises a cylindrical photoelectric cathode surrounded by acoaxial grid type anode within an evacuated container or vessel. Meanssuch as a permanent bar magnet located inside of the cathode or solenoidwindings positioned exteriorly to the container produce a magnetic fieldin the space between the cathode and the anode with the lines of force,substantially parallel to the axis, common to the electrodes. Thismagnetic field causes the electrons to traverse curved paths ,and returnto the cathode under suitable cathode to anode voltages. When acyclically pulsating voltage of suitable frequency is impressed betweenthe cathode and the anode, multiplication by repeated bombardment takesplace during the part of the cycle in which the cathode becomes less andless negative with respect to the anode, and during the other part ofthe cycle in which the cathode becomes more and more negative withrespect to the anode the electrons are collected at the anode. Thecathode is an emitter of both photoelectric and secondary electrons.

This device with the central cylindrical cathode surrounded by the openmesh anode and means for producing the magnetic field designed so as notto interfere with the illumination of the cathode may be operated as anelectron multiplier photoelectric tube, and also as a high frequencyoscillator either modulated by light or independently of light. In thelatter case the cathode need only to be so constructed as to be a goodsecondary emitter of electrons independently of its photoelectricsensitivity.

A more detailed description of the invention follows:

Fig. 1 is a perspective side and partial sectional view of an electrondischarge device employing solenoid windings for producing a magneticfield;

Fig. 2 is a perspective side and partial sec tional view of an electrondischarge device employing a permanent magnet for producing a magneticfield;

Fig. 3 is a longitudinal sectional view of the electrodes and thepermanent magnet of the arrangement shown in Fig. 2;

Fig. 4 is a circuit arrangement of the electron discharge deviceoperating as a photoelectric tube;

Fig. 5 shows diagrammatically the path of an electron shot out of andreturned to the cathode and paths of resulting secondary electrons;

Fig. 6 is a graph showing the action of the electron discharge deviceduring two cycles of oper-- ation;

Fig. 7 is a circuit arrangement of the electron discharge deviceoperating as an oscillator; and

Fig. 8 shows the electron discharge device equipped with a parabolicreflector for concentrating light rays on the cathode.

Similar reference characters referv to corresponding parts in thedifferent figures of the drawing.

Fig. 1 shows a perspective side and partial sectional view of theelectron discharge device employing solenoid windings for producing themagnetic field. A container or envelope I ll of any suitable materialsuch as glass, from which the gas has been highly exhausted, containsthe 'electrodes and their supports. The electron emitting cathode 20 inthe form of a small hollow cylinder is centrally'mounted within thecontainer ill. The cylindrical anode 30 is co axially mounted around thecathode and so formed as to permit light to reach the cathode fromsubstantially all directions. Suitable leadin wires 2| and 3i are sealedto the stem of the container and connect with the electrodes 20 and 30,respectively. The cathode 20 is made of material to readily andefiiciently emit electrons when exposed to light and also when bombardedby other electrons to amply emit secondary electrons. It may be ofsilver coated with the usual caesium-oxide silver surface and is hereshown in the form of a small elongated hollow cylinder with a relativelythin wall. The anode II is made of any suitable conducting butnon-magnetic material such as copper, aluminum or the like and may befabricated in open mesh grid design in the form of a squirrel cage asshown in the drawing, or with wire mesh, or as a latticed cylinder, thedesign being such as to have a large amount of open space for permittingthe light to pass within to the concentrically positioned cathode 2..Both the cathode II and the anode 30 preferably employ a relativelysmall amount of materialso as to reduce to a minimum the problem ofremoving the occluded gas. A uniform magnetic field with the lines 01'force substantially parallel with the axis of the electrodes is producedby means of spaced solenoid coils ll and 42. The coils are of such sizeand so spaced as to readily permit light flux to reach the cathode 20from all radial directions.

An electron discharge deviceconstructed substantially as shown in Fig. 1and tested in actual operation comprised a thin light sensitive tubularcathode approximately one-eighth inch in diameter and one and one-halfinches long concentrically surrounded by a "squirrel cage shaped anodeone inch in diameter and one and fiveeighths inches long. The evacuatedglass container in which the electrodes were mounted was approximatelytwo inches in diameter and four and one-half inches long. Otherdimensions, however, may be employed.

Fig. 2 shows a perspective side and partial sectional view of anelectron discharge device which is similar to Fig. 1' with the exceptionthat the magnetic field is produced by a permanent magnet instead of bysolenoid windings. While the permanent magnet ll might be positionedwithin the evacuated chamber and inside of the cylindrical cathode ll ofthe structure shown in Fig. 1, it is preferable to modify the containingvessel I! by adding an exteriorly opening and internally extendingtubular reentrant stem ll around which, but spaced therefrom by micaspacer washers l2, the cathode 20 is positioned within the container andinwhich the permanent magnet 40 is positioned in the exteriorly openportion of the reentrant stem. This permits the initial insertion or thereplacement of the permanent magnet at any time and also obviates theremoval of occluded gas in the permanent magnet which would be necessarywere the permanent magnet placed in the cathode during construction inthe arrangement shown in Fig. 1.

Fig. 3 shows more in detail the relative positions of electrodes 20 and30 and the permanent magnet 40 of the arrangement shown in Fig. 2, drawnto a difl'erent scale, and especially the di rection of the magneticlines of force shown by the dashed lines. The portion of the reentrantstem ll surrounding the magnet II and the spacer washers l2 between thestem II and the cathode III are also shown. The magnetic lines of forceoccupy the space between the two electrodes and the direction of thelines of force is substantially perpendicular to the radii of thecathode. With the magnetic lines of force having such direction,electrons shot out from the cathode toward the anode must transverselycross the lines of force. The lines of force so positioned consequentlytend to bend the electrons back toward the cathode over looped paths asdiagrammatically shown in Fig. 5. Positioning the open mesh cylindricalanode ll concentrically with the cylindrical cathode 20 has an advantageover the reverse arrangement in which the anode is positioned in thecenter in that the light can be directed from all sides of the tube, andhigher frequencies can be developed because the electrons do not travelthrough regions of equipotential electric force.

Fig. 4 shows a circuit arranged for operating the electron dischargedevice of either Fig. 1 or Fig. 2 as a photoelectrictube employingsecondary electron emission to increase the output. A source ofalternating current impresses through the transformer I a sine or othersuit- .ably shaped wave upon the circuit to cause proper potentialvariation of the cathode 20 with respect to the anode 30 to causeelectron multiplication. The anode I0 is maintained at a high positivepotential above ground by the direct current source ill. A loadimpedance 10 completes the output circuit, and connections H and 12across this impedance provide means for talcing oi! the amplified signalvariations controlled by the light flux from any suitable source I"exciting the cathode. A steady magnetic field of proper strength isprovided either by solenoid windings or by a permanent magnet whoselines of force are substantially parallel with the common axis of theelectrodes.

The probable way in which the circuit of Fig. 4 functions, is that theprimary electrons, initially photoelectrically produced by illuminationof the cathode 20, first loop back toward the cathode during theinterval when the cathode is becoming less and less negative withrespect to the anode, so that they strike the cathode with sufflcientforce to emit secondary electrons. These secondary electrons, which aremore numerous than the primary electrons which produce them, follow asimilar cycle looping back to the oathode under the combined electricand magnetic fields to repeatedly strike it and cause the emission ofstill greater numbers of secondary electrons. This multiplying actiongoes on until the potential of the cathode no longer approaches that ofthe anode during the flight of the electrons over the looped paths,namely, until the sine wave passes its crest and the cathode begins togo increasingly negative with respect to the anode. During this secondhalf cycle the electrons spiral out to the anode where they arecollected. In other words the high frequency alternating potential waveof proper value impressed across the electrodes of the tube causesduring each alternate half cycle electron multiplication to repeatedlytake place by impact of the electrons against the cathode, and duringthe other alternate half cycle permits electrons to be collected by thepositively charged anode. 0n the multiplication half cycle while theelectrons complete one loop back to the cathode, the oathode changespotential with respect to the anode enough so that the electrons strikeit with 50 to volts energy to cause their multiplication.

On the collection half cycle the electrons spiral over to the anode andare collected, as stated above.

The above described probable functioning of the circuit of Fig. 4 willnow be described in more detail by reference to Figs. 5 and 6.

Fig. 5 diagrammatically shows the looped paths of a primary electron andthose of the secondary electrons after their generation. As heretoforestated, the magnetic lines of force set up in the space between theelectrodes 20 and 30, by either the solenoid windings or the permanentmagnet, as illustrated in Fig. 3, tend to cause the electrons emittedfrom the cathode 26 to return to it over looped paths asdiagrammatically shown in Fig. 5. An electron emitted from the cathodeat a forcefully returns over a looped path at b and causes the emission.of secondary electrons which likewise return over looped paths tobomhard the cathode at c and cause the emission of more secondaryelectrons. Many such actions are going on at each instant during thegeneration of secondary electrons. The magnetic field and theelectrostatic field operates on both the primary and the secondaryelectrons to cause them to return to the cathode.

Fig. 6 shows the potential variations of the cathode with respect to theanode as following a sine wave. A steady positive potential V1 from asource 5d applied between the cathode and the anode holds the anode at ahigh positive potential V1 and a superimposed alternating potentialhaving a peak amplitude about equal to the steady potential V1 from asource 56 causes potential variations of the cathode with respect to theanode over a range of about twice the steady potential V1. At the pointP1 on the curve, the cathode and the anode are at nearly the samepotential, while at the point P2 the cathode is negative with respect tothe anode by about twice the steady potential V1. During a part of eachcycle as shown by the vertically shaded portions of this curve, electronmultiplication is taking place and during the other part as shown by theobliquely shaded portions the multiplied electrons are drawn to andcollected by the anode. The movement of the electrons duringmultiplication follows looped paths from and back to the cathodesomewhat as shown in Fig. 5, and during collection by the anode theelectrons spiral out to it,

As shown in Fig. 5, an electron emitted from the cathode at a willstrike at b causing the release of more than one secondary electron.These secondary electrons will go on to 0, etc., thus greatlymultiplying or amplifying the output current. As the cathode becomesless and less negative with respect to the anode during the time from mto n, as shown by the curve of Fig. 6, multiplication is taking place.The voltages of the cathode corresponding to points a, b, 0, etc., maybe considered as shown by points 1, 2, 3, etc. on the curve. During themultiplication half cycle from m to n the electrons start out overlooped paths and when they get back to the cathode, its voltage hasbecome more positive and consequently the electrons hit the cathode withenergy equal to the difference between the cathode potential at the timethey left it and that obtaining when the electrons strike it again. Thispotential difference must be at least equal to that necessary to causethe secondary to primary electron generation ratio to be greater than 1.This continues throughout the multiplication half cycle, the loopsgetting smaller and smaller because of the decreasing voltage differenceproduced by the superimposed alter-- nating potential wave until finallywhen the potential of the cathode is about the same as that of theanode, the electrons will cease to return to the cathode, and will thenstart to spiral out to be collected by the anode, this collection takingplace during the period n to m of increasing potential difference untilthe maximum is again reached and the cycle starts over again.

Fig. 'l is a circuit arranged for operating the electron dischargedevice as an oscillation generator. The cathode 20, a good emitter ofsecondary electrons, and the anode at are connected by an externalcircuit including a tuned circuit consisting of an inductance 8i and avariable capacitance 82 in shunt and a steady source of high potential50 having its positive side connected with the anode 30. The source ofpotential 5B is preferably shunted by a capacitance 5!. Outputconnections with the oscillator circuit may be obtained through thetransformer action on winding 83 positioned in inductive relationshipwith the inductance 8| of the tuned circuit. As heretofore stated, whenthe electron discharge device is operating as an oscillator the cathodeneed not be light sensitive, but must be a good emitter of secondaryelectrons. However, its output may be modulated by light by employing alight sensitive cathode or electrically by well-known circuitconnections. Operation of the oscillator may be initiated by causing asharp electric disturbance in the circuit. Such a shock may beaccomplished in a number of ways such as by quickly opening and closingthe primary oscillating circuit. In this discharge device higherfrequencies can be developed as the electrons do not travel throughequipotential electric fields of force. This oscillator is of verysimple construction and is capable of oscillating over a very wide bandof frequencies.

Fig. 8 shows the electron discharge device equipped with a parabolic orother suitable reflector for concentrating the light rays on the cathodeof the tube. The reflector H0 and the tube iii are so positioned thatthe cathode of the discharge device is positioned in the focus of thereflector. The parabolic reflector may be either circular with its axisand thatof the cathode of the tube coinciding as shown in Fig. 8, or theparabolic mirror may be cylindrical with its axis and that of thecathode of the tube also coinciding. Such a reflector has the advantageof concentrating the light flux around the cathode.

What is claimed is:

1. An electron discharge device comprising an evacuated container, acylindrical cathode the convex surface of which is capable of emittingsecondary electrons, a cylindrically shaped electrode surrounding andcoaxial with said cathode, means for producing a magnetic field in thespace between said cathode and electrode substantially parallel to theaxis of said cathode, and means to apply a pulsating potential betweensaid cathode and electrode of such value and frequency that some of theelectrons emitted from said cathode return to said cathode withsuiilcient energy to cause the emission of additional secondaryelectrons.

2. An electron discharge device comprising an evacuated container, acylindrical cathode having a convex surface capable of emittingsecondary electrons, a cylindrically shaped electrode surrounding andcoaxial with said cathode, a permanent magnet concentrically positionedwithin said cylindrical cathode for producing a magnetic field in thespace between said cathode and electrode substantially parallel to theaxis of said cathode, and means for applying a pulsating potentialbetween said cathode and electrode of such value and frequency that someof the electrons emitted from said cathode return to said cathode withsuflicient energy to cause the emission of additional secondaryelectrons.

3. An electron discharge device comprising an evacuated container, acylindrical cathode the convex surface of which is capable of emittingsecondary electrons, a cylindrically shaped electrode surrounding andcoaxial with said cathode, spaced solenoid windings for producing amagnetic field in the space between said cathode and electrodesubstantially parallel to the axis of said cathode and arranged tofreely admit light to said .cathode, and means to apply a pulsatingpotential between said cathode and said electrode of such value andfrequency that some of the electrons emitted from said cathode return tosaid cathode with sufiicient energy to cause the emission of additionalsecondary electrons.

4. An amplifying photoelectric system comprising an electron dischargedevice including a cylindrical cathode having a convex light sensitivesurface capable of emitting secondary electrons, a cylindrical grid-likeanode coaxially surrounding said cathode, and an evacuated containerenclosing said cathode and said anode, means for producing a magneticfield in the space between said cathode and said anode with the lines offorce substantially parallel to the axis of said cathode, an externalcircuit connecting across said cathode and said anode and including anoutput impedance, a source of steady potential, and a source ofalternating potential, and output circuit connections leading from saidcircuit.

5. A high frequency oscillation generator comprising an electrondischarge device including a cylindrical cathode having a convex surfacecapable of emitting electrons, a cylindricalanode coaxially surroundingand having openings permitting light to impinge upon the convex surfaceof said cathode, and an evacuated container enclosing said cathodeandsaid anode, means for producing a magnetic field in the space betweensaid anode and said cathode with the lines of force substantiallyparallel to the axis of said cathode, an external circuit connectingacross said cathode and said anode and including a tuned network and asource of steady potential maintaining a difference in potential betweensaid cathode and said anode, and an output circuit coupled with saidexternal circuit.

6. An electron discharge device comprising an evacuated container, anelongated cathode which is capable of emitting secondary electrons, anelongated anode spaced-from and generally parallel to said cathode,means for producing a magnetic field in the space between said cathodeand said anode with lines of force substantially parallel to the axis ofsaid cathode, and means for applying a pulsating potential between saidcathode and said anode of such value and frequency that some of theelectrons emitted from said cathode return to said cathode withsufficient energy to cause the emission of additional secondaryelectrons.

'7. An electron discharge device for causing repeated electronmultiplication from the same cathode, comprising an elongated cathodeelectrode capable of emitting secondary electrons, an anode electrodeencircling said cathode, means for producing a steady magnetic field inthe space between said electrodes with the lines of magnetic forcesubstantially paralleling the axes of said electrodes, means forproducing an electrostatic field between said electrodes, and

means for cyclically varying said electrostatic field over such a rangeof intensities that electrons emitted from said cathode return theretowith suillcient energy to cause repeated electron multiplication duringone part 01 each of said cycles and to cause the electrons to collect onthe said anode during another part of each of said cycles.

8. An electron discharge device comprising an evacuated container, asingle cylindrical cathode having a convex surface capable of emittingsecondary electrons, a single cylindrically shaped electrode surroundingand coaxial with said cathode, and a permanent magnet concentricallypositioned within said cylindrical cathode for producing a magneticfield in the space between said cathode and said electrode with thelines oi! force substantially parallel to the axis of said cathode.

9. A photoelectric tube comprising a hollow cylindrical cathode havingan electrically coextensive convex surface capable of emitting secondaryelectrons, an anode coaxially surrounding said cathode and having anelectrically coextensive surface, an evacuated container enclosing saidcathode and said anode, and a magnet positioned within the said cathodeand producing a magnetic field in the space between said anode and saidcathode with the linesof force substantially parallel to the axis ofsaid cathode.

10. A photoelectron multiplying device comprising a photosensitivecathode capable of emitting primary electrons when excited by radiantenergy and capable of emitting secondary electrons when bombarded byelectrons, an electrode spaced from said cathode, means for producing amagnetic field between said cathode and said electrode having lines offorce generally parallel to the surface of said cathode, means includingsaid magnetic field for bombarding said cathode by said primaryelectrons with suiiicient force to cause emission of secondary electronsand said secondary electrons in turn to successively bombard the samecathode thereby producing more secondary electrons, and means includingsaid cathode and said electrode for'impressing a varying electrostaticfield between said cathode and said electrode with its lines of forcegenerally normal to those of said magnetic field.

11. An electron discharge system comprising a single cathode capable ofemitting primary and secondary electrons, a single electrode coaxiallypositioned with reference to said cathode, an

evacuated container enclosing said cathode and said electrode, means forproducing a magnetic field in the space between said cathode and saidelectrode with the lines of force substantially parallel to the axis ofsaid cathode, means for maintaining all 01' the surface of said electronemitting element at any instant at substantially the same electricalpotential and polarity with reference to that of said electrode, meansfor producing and for cyclically applying a varying potential betweensaid cathode and said electrode for causing some of the electronsemitted from said cathode to return thereto with sufficient energy tocause the emission of additional secondary electrons during one part ofeach of said cycles and to cause the electrons to collect on saidelectrode during another part of each of said cycles.

ALBERT M. SKELLE'I'I.

